For effective management of bacterial infection, early diagnosis of the presence of an infection/colonisation and swift treatment is essential. A particular problem in clinical environments such as hospitals is detecting, isolating, and treating patients and health workers carrying ‘superbugs’ such as MRSA (methicillin-resistant Staphylococcus aureus) before they come into contact with vulnerable patients. The prevalence of infections caused by MRSA has been increasing for several years in many countries around the world.
Reports from the UK National Audit Office state that at any one time, 9 percent of NHS hospital patients are suffering from an infection such as MRSA, acquired whilst in surgery or as an inpatient on the hospital wards. These ‘nosocomial’ infections affect 100,000 people annually, costing the National Health Service £1 billion (1.5 bn), and causing up to 5,000 deaths. The US Centre for Disease Control and Prevention estimates that between 60,000 and 80,000 Americans die each year from nosocomial infections, and the cause in the majority of cases is S. aureus. 
Some strains of MRSA are particularly successful at spreading between patients and may also spread between hospitals, for example when colonised patients or staff move from one hospital to another. These strains are known as epidemic MRSA (EMRSA). During the 1990s there was a marked increase in infections caused by MRSA in hospitals in the UK due to the emergence and spread of two particular stains of EMRSA known as EMRSA-15 and EMRSA-16. In 2002, a new epidemic strain, EMRSA-17, was described in the UK. This strain is more resistant than any previous UK EMRSA strains.
Older methods of screening are too slow to effectively prevent the spread of hospital-acquired infections and can be very expensive and labour-intensive. Some existing methods are also inaccurate and may suffer from problems with specificity and sensitivity. Laboratory screening for MRSA is therefore a complex balance between speed of result, sensitivity, specificity and cost.
Standard methods for diagnosis of MRSA involve overnight culturing of the bacteria followed by visual identification and verification of MRSA colonies. Agar plate-based methods are the most common, using either selective or non-selective media. Broths or slopes (which may also be selective or non-selective) can also be used. These methods have been practised for many years, and are generally quite specific and sensitive. However, they can provide answers only after 3-5 days, which is of little use in a hospital setting, from the perspective of infection control.
As a result, newer methods, in particular PCR based methods, have been developed with the aim of providing results more rapidly. PCR results can generally be obtained in one working day, compared to three working days for culture methods, although this is still too long if the spread of the bacteria is to be effectively controlled.
The ‘first generation’ of PCR-based methods (e.g. References 1-4) rely on the presence of a combination of the methicillin resistance gene, mecA, with one or more other genes specific to S. aureus (e.g. femA or nuc). These methods have major drawbacks in the form of false positive rates, costs, and lack of automation, and are mostly only suitable for use with pure cultures, not screening of clinical swabs.
The more recently developed generation of PCR-based methods (e.g. References 5-8) have had some success. For example, the IDI-MRSA PCR test, in which a single genetic sequence is detected specific to MRSA, is now approved for use in nasal swabs for colonisation. These methods still suffer from problems, however. Using nasal swabs only, they will only catch 50% of the patients who are colonised, and none of those with bacteraemia.
Other methods which have been developed include the latex agglutination method (Reference 9). The most commonly used commercial test relies on the presence of Protein A and PBP2A which, combined together, point toward MRSA. However, this test has sensitivity and specificity problems.
There is a need for a new method for the rapid diagnosis of bacterial infections, such as MRSA, which overcomes the sensitivity and specificity problems of currently available methods, and can be performed quickly, easily and at low cost.